Television or like system and apparatus



Dec, 13, 1932. .1. 1.. BAiRo 1,890,558

wmavrszon 0R LIKE SYSTEM AND APPARATUS Filed April 8. 1929 z sheetwsne t 1 13, 1932. J iR I 1,890,558

TELEVISION 0R LIKE SYSTEM AND APPARATUS Filed April 8, 1929 2 Sheets-Sheet 2 M,MM

Patented Dec. 13, 1932 UNITED STATES JOHN LOGIE BAIRD, OF LONDON, ENGLAND, ASSIGNOR TO TELEVISION LIMITED, OF

PATENT OFFICE LONDON, ENGLAND, A BRITISH COMPANY TELEVISION OR LIKE SYSTEM AND APPARATUS Application filed April 8, 1929, Serial No. 353,632, and in Great Britain April 16, 1928.

This invention is for improvements in or relating to television or like systems and apparatus for transmitting to a distance views, scenes, images or the like, hereinafter, for the sake of brevity, referred-to as pictures, in which systems and apparatus the transmitting and receiving devices must revolve in synchronism.

One feature of the present invention is the method, in a television system or the like, of

a maintaining synchronism between a transmitter and a receiver which consists in transmitting a distinctive signal, receiving it on the circuit of the receiver, coupling the re ceiver circuit by a synchronizing switch to a correcting device in such manner that when the receiver is out of phase the signal operates the correcting device but when the receiver is in phase the signal is not applied to the correcting device.

A further feature of the invention is a method of the kind above set forth which is characterized by the fact that the distinctive signal is obtained by the contrast of the picture signal with a signal which is weak compared with the picture signal or is of zero value.

The invention comprises also as a feature, a synchronizing mechanism in a television system or the like, which mechanism comprises devices (including for example a commutator or a light-sensitive cell) for transmitting periodically to the receiver a distinctive signal, and means (for example, a commutator) running in synchronism with the receiver and operating by reason of said distinctive signal, if the receiver is not in synchronism with the transmitter, to feed current to a device (for example a winding of a relay or the like or a winding on the field magnet of a motor), by the action of which device the synchronism of the receiver is restored.

The above and other features of the invention pointed out in the appended claimingclauses will now be described in connection with systems and apparatus embodying the features of the invention in convenient forms.

In the accompanying diagrammatic draw- 1ngs Figure 1 shows one form of synchronizing mechanism including a commutator at a transmitting station;

Figure 2 shows another form of commutator synchronizing mechanism at a transmitting station;

Figure 3 shows a third form of such mechanism, also at a transmitting station;

Figure 4 illustrates a synchronizin device at the transmitting station in which a light-sensitive cell is used;

Figure 5 shows one form of synchronizing mechanism at the receiving station;

Figure 6 shows another form of such mechanism at the receiving station;

Figure 7 illustrates a construction in which one commutator controls two relays or like devices;

Figure 8 illustrates a wedge-shaped or stepped band that may be employed on the, picture for synchronizing purposes;

Figure 9 is a development of a commutator that would produce the band of Figure 8;

Figure 10 illustrates synchronizing mechanism comprising three relays adapted to 0p erate in a predetermined manner;

Figure 11 illustrates a commutator the brush gear of which is adapted to be shifted by a governor device;

Figure 12 shows a further form of synchronizing mechanism at the receiving station;

Figure 13 illustrates the application of a form of the invention similar to-that shown in Figure 7 to a receiving circuit; and

Figure 14 shows a modified synchronizing control in a receiver.

In constructions according to the present invention there may be employed at the transmitting station an exploring disc 2, Figure 1, perforated with a spiral series of holes consisting of, say, thirty holes. In conjunction with this disc and revolving in synchronism with it (say, mounted at the bottom of the same shaft) there is a commutator 4 having thirty segments, each segment respectively corresponding to one hole of-the disc 2 and The segments are arranged to make contact between two brushes 6 and 8 through which passes the image current modulated by a light-sensitive cell 10. The image current is thus interrupted at a certain point in each line of the picture.

It will be understood that the brushes 6,

- 8 are so arranged that at the time the picture signals are flowing, each brush will rest upon the same segment of the commutator 4,

. object, that is, during the period of transition between the cessation of the transmission of'light from one element of the scanning device and the commencement of transmission by a consecutive element of the device. This period is technically known as the underlapperiod and is well known in the art of transmission of still pictures and facsimile reproduction. The image current is amplified by a suitable amplifier 12 from which the current passes to the receiving station. By means of this arrangement a distinct synchronizing signal is transmitted at a frequency of repetition definitely related to the scanning frequency. This signal may be utilized to actuate any suitable mechanism at the receiving station for controlling the scanning speed.

Instead of interrupting the current at a certain point in each line of the picture, relatively powerful signals may be sent to the receiver at that time; this may be effected by applying a large positive bias to a three-electrode valve or by short-circuiting such a valve. The latter method is illustrated in Figure 2. In this figure there is a commutator 14 in which the segments'are narrow compared with the insulating material between them. Brushes 16, 18 are arranged to make contact simultaneously with each segment as it passes beneath them in the rotation of the commutator. The brushes 16, 18 are in a circuit which includes the anode and filament of a valve 20, so that this circuit is closed when the brushes make contact with a segment, resultng in a relatively large current from the high tension supply 22 flowing momentarily in the circuit, the amount of this current being suitably limited by a resistance 24 inserted in the circuit. The input terminals from the light-sensitive cell (not shown) are indicated at 26.

Figure 3 illustrates'the case where a large positive bias is applied to the valve. In this figure the brushes 28 bear upon a commutator 30, which, like the commutator 14 of Figure 2, has its segments narrow relatively to the insulation between them. The brushes 28 are in a circuit'which includes the grid and filament of a valve 32 together with a source of potential 34 to produce the necessary positive bias.

Figure 4 illustrates a construction where in place of the commutator shown in Figures 1 to 3, a light-sensitive cell 36 is employed to give a synchronizing signal; in this form an exploring device 38 is used which in addition to the normal spiral arrangement of holes 40, has a series of radial slots 42, one corresponding with each of the holes in the spiral 40. A light-source (not shown) is placed upon the side of the disc opposite that on which the cell 36 is located and activates that cell when a slit is on the line joining the light and cell, the latterbeing suitably screened to ensure activation thereof only at such time as each of the holes 40 moves out of, or the succeeding hole 40 moves into the field of vision. The cell activated by light from the object to be transmitted is indicated at 44. The modulated currents from the cells 36 and 44 respectively are amplified by amplifiers 46 and 48 and then fed to theline on which signals in any suitable manner are transmitted to the receiving station.

At the receiving station there is provided a commutator running in synchronism with the exploring disc which reconstitutes the transmitted picture, for instance by mounting the commutator and disc on a common shaft. One form of the synchroniz ng mechanism is. shown in Figure 5, in which a commutator 50 connects two brushes 52 which are in series with the varying light-source 54 used to produce the picture. Thus say, at the bottom of each line of the reproduced picture the commutator breaks the contact between the two brushes 50, and as a relay 56 is shunted across the brushes 52, the relay operates every time the intense synchronizing signal is sent by the transmitter during the period the circuit is broken by the commutator 50. The ordinary signals from the picture are not sufli ciently intense to operate the relay 56, or the arrangement may be such that on each side of the intense signal or on one side only, the image is made dim (for example by a suitable mask or light-filter) so as to eliminate or minimize any chance of the picture signals interfering with the synchronizing currents. A valve which amplifies the received signals is indicated at 58.

A modification of this arrangement is shown in Fig. 6 of the drawings. In this figure a relay 60 is connected between the filament and plate of a valve 62 and in series with the relay are two brushes 64 which are short-circuited by a commutator 66, which is driven in synchronism with the exploring disc, at a position corresponding to the hottom of each line of the picture. The plate of the valve 62 is connectedto the varying light-source 68, which if a glow-discharge lamp should be shunted with a high resistance so that in darkness there is still current passing. It will be observed that in this arrangement, as in the arrangement shown in Figure 5, the relay is periodically connected to the receiving circuit so as to be affected by received signals, the periodicity of this connection coinciding with or hearing a definite timed relation to the line scanning frequency of the receiver exploring disc.

In operation, the receiving machine is run up to speed when an image is seen with a dark line or an intense white line, as the case may be, at the top and bottom. There is a datum line on the machine such as a pointer or other indicator in line with the brushes, and when the dark line or white line is opposite the pointer, the synchronizing signal controls the relay. The relay is switched in when the pointer points at preferably the part of the picture just before the dark or bright line, and the receiving machine is adjusted so that it travels slightly faster than the transmitting machine, thus causing the line to creep up towards the pointer. lVhen the line coincides with the pointer an impulse is sent through the relay which operates and brings a retarding influence to bear upon the receiving machine thus keep ing it in step with the transmitter. The relay is of course so constructed as to respond to intense signals or signals of greatly reduced intensity depending on whether the synchronizing signal is of an intense or reduced nature.

- Figure 9, which shows the commutator surface developed.

In operation, the correcting lines partially overlap the datum mark; if the receiver runs fast the overlap increases and the retarding correct-ion also increases: if it runs slow the overlap diminishes and fewer correcting retarding impulses are given. There is thus a continued governing action and hunting is reduced or eliminated.

In connection with the receiver, there may be employed a centrifugal governor of the friction type, or other known type giving a gradual speed control, in combination with the communtator control of the present invention, as by such a combination only a relatively small controlling force is required from the relay.

Further at the receiving station, as shown in Figure 7, the commutator 72 may be provided with two sets of brushes 74, 76, each set controlling a relay (not shown) and so arranged that when the commutators at both stations are in step the synchronizing impulses will arrive at the receiving station when both sets of brushes 74, 76 are upon the insulation between the commutator segments and no speed-correcting effect will take place. But if the receiver commutator 72 overruns 0r lags behind the transmitter commutator, one or the other of the relays is actuated to retard or accelerate, as the case may be, the running apparatus at the receiving station to bring the commutators in step again. The

receiving commutator 72 will preferably have moves up the. impulses are in the order of 3, 2, 1, so that the upward motion gives a different signal from the downward motion and these signals are used to give correcting impulses.

To obtain this effect the apparatus illustrated in Figure 10 may be employed. This apparatus comprises three pairs of brushes 78, 80, 82, bearing upon a commutator 84. The brushes 78 are in series with a relay 86 in a circuit in which the synchronizing signals flow. The brushes 80 are in series with a relay 88 and the brushes 82 in series with a relay 90 and the relays 88 and 90 together with their respective brushes are in parallel in a circuit containing a source of power 92 that is made or broken by the operation of the relay 86. With the parts in the position pulses in the order of say 1, 2, 3. If the image shown in Figure 10 the synchronizing signal will flow through the relay 86, but the closing of that relay will not affect the relays 88 and 90, for brushes 80 and 82 are resting upon the insulation of the commutator. This is the position of the parts when the machines are running in step. If the receiver advances, i. e. if the commutator 84 moves to the left in Figure 10, the brushes 80 will bear upon a metal part of the commutator so that when the relay 86 is energized by a synchronizing signal the relay 88 will be operated to give a retarding impulse. On the other hand, if the receiver is lagging the relay 90 will be actuated and will cause an accelerating impulse to be given to the receiver. By this arrangement no action can be given to relay 88 or relay 90 unless the master relay 86 is operated,

thevnature of the correcting impulse depend-' 86 is followed by that of 88 or by that of 90.

Under certain circumstances it may be desirable to employ more than one set of the three relays 86, 88, 90, with their respective sets of brushes 78, 80, 82, the sets of brushes being spaced along the surface of the com mutator. With such an arrangement if, say, the receiving machine was lagging and the relay 90 of the middle set of relays imparted an accelerating impulse to the receiver such that it passed the retarding relay 88 without its acceleration being completely stopped, the next action upon the receiver would be by the retarding relay of the first set. Thus, the receiver if unduly accelerated would have two retarding impulses applied to it. On the other hand if the retardation of the receiver was unduly excessive it would have two accelerating impulses imparted to it in succession. This is due to the fact that no impulse, either retarding or accelerating, can be given to the receiver until the master relay 86 has operated.

In Figure 11 is illustrated a construction whereby a brush 94 is operatively connected with a governor 96 driven by thereceiving machine. The connections between the brush and the governor comprises a guided rod 98 connected with an elbow lever 100 engaging the usual sleeve of the governor. The commutator 102 in this case-has its segments tapered and so arranged that as the brush 94 is moved across the commutator surface by an increase or decrease in the speed of the receiver, so the period of time within which the brush is operative to effect correcting signals is correspondingly varied. Convenient ly the correcting current is fed through a re lay device the operation of which is dependent upon the quantity of current flowing.

The relays above described may bring about variation in the s eed of the receiving apparatus in any suitable manner, for example by cutting out, or introducing, a resistance in the field magnet circuit of the motor driving the receiver, for instance as illustrated in Fig. 13, or by cutting in or out an auxiliary field winding of the motor. An example of the last named method is illustrated in Figure 12. wherein a commutator 104 has brushes 106 in engagement with it which are arranged periodically to short-circuit an auxiliary field winding 108 of the motor.

The field winding may be shunted by an appropriate condenser 110 and in addition. if desired, a relay 112 may also be controlled by the commutator 104 and brushes 106 to assist in controlling the speed of the motor.

In any of the foregoing examples where it is found desirable or necessary, the lightsource of the receiver may be shunted by any appropriate resistance and the relays shunted by a suitable condenser.

In a further modification the receiver moi tor, which may be a direct-current shunt-mo- In this modification each time the transmitter commutator reduces the picture current to a low value or to zero, (for example at the bottom of each band of the picture) the polarized relay acts upon the phonic wheel and tends to keep it and the motor coupled to it, in synchronism with the transm tter. The commutator also, if transmitter and receiver become out of step, acts to feed the picture signals to the relay which thereupon causes the speed of the receiver to be accelerated or retarded, as the case may be, to bring the receiver into phase with the transmitter. This combined control upon phonic wheel and motor tends to keep the receiver both in phase and in synchronism with the transmitter. Any other suitable synchronous motor may be used instead of the phonic wheel above mentioned.

It will be appreciated that a well-marked synchronizing signal is obtained by the contrast between the average picture current and the zero or substantially zero current which is responsible for the appearance of the black line on the picture, and that this s gnal is sufliciently distinctive from the picture signal to give a distinctive frequency to the vibrating member of the phonic wheel which may also be tuned or adjusted to this frequency. The inclusion of the neon tube in the circuit accentuates the distinctive signal owing to the fact that at voltages below its striking voltage the neon tube becomes approximately a non-conductor so that at the b ack line on the picture the current is substantially complete 1y interrupted which would not be the case if a device containing merely ohmic resistance replaced the neon tube. If the commutator or its relay becomes inoperative due to shortcircuiting or the like the phonic wheel retains the apparatus in approximate synchronism.

I claim 1. In a system for sending light images having a transmitting device and a receiving device. each of said devices including means for exploring the field of vision, the combination with motors for driving said exploring means, of a commutator operating in timed relation with the motor associated with the transmitting device. and means associated with the commutator segments for effecting a substantial variation in the intensity of the transmitted signal at predetermined intervals, the commutator segments being so spaced as to enact with said means when said exploring means is operating adjacent one side of the field of vision to produce a band of distinctive light intensity across one side of the received image, mechanism controlled by the intensity of the received signal for controlling the speed of the motor associated with the receiving device, and means for associating the received signal with said mechanism at intervals corresponding to said first mentioned intervals, said means including a commutator driven in timed relation to the motor associated with the receiving device, and means associated with said commutator to feed the received signal to said mechanism, whereby the phase relationship of said first and second mentioned intervals may be utilized to effect synchronization of the transmitting and receiving devices.

, 2. In a system for sending light images having a transmitting device and a receiving device, each of said devices including means for exploring the field of vision, the combination with motors for driving said exploring means, of a commutator operating in timed relation with the motor associated with the transmitting device, and means associated with the commutator segments for efiecting a substantial variation in the intensity of the transmitted signal at predetermined intervals, the commutator segments being so spaced as to coact with said means when said exploring means is operating adjacent one side of the field of vision to produce a band of distinctive light intensity across one side of the received image, and said segments being of varying Width whereby the roduced band is substantially wedge'shape mechanism controlled by the intensity of the received signal for controlling the speed of the motor associated'with the receiving device, and means for associating the received signal with said mechanism at intervals corresponding to said first mentioned intervals, whereby the phase relationship of said first and second mentioned intervals may be utilized to efl'ect synchronization of the transmitting and receiving devices.

In testimony whereof I afiix my signature.

- JOHN LOGIE BAIRD. 

